<!DOCTYPE html>
<html>
<head>
    <meta charset="UTF-8">
    <title>画一个红色的圆点</title>
</head>
<body>

    <canvas id="myCanvas" width="200" height="100"
            style="border: 1px solid darkgrey;">
    </canvas>

    <script type="text/javascript">
        var c = document.getElementById("myCanvas")
        var cxt = c.getContext("2d");
        cxt.fillStyle = "red";
        cxt.beginPath();
        cxt.arc(70,18,15,0,Math.PI*2,true);
        cxt.closePath();
        cxt.fill();
    </script>
</body>
</html>